1. Field of the Invention
This invention relates to a measuring apparatus such as a surface plasmon resonance sensor for quantitatively analyzing a material in a sample on the basis of generation of surface plasmon, and more particularly to a measuring apparatus in which a light beam is caused to reflect in total reflection at the interface between metal film or a clad layer in contact with a sample and a dielectric body to generate evanescent waves and the sample is analyzed on the basis of change in intensity of the light beam reflected in total reflection.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon resonance sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called “Kretschmann configuration” is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
The surface plasmon resonance sensor using the Kretschmann configuration basically comprises a dielectric block shaped, for instance, like a prism, a metal film which is formed on one face of the dielectric block and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block at various angles of incidence so that total internal reflection conditions are satisfied at the interface of the dielectric block and the metal film and various angles of incidence of the light beam to the interface of the dielectric block and the metal film including an angle of incidence at which attenuation in total internal reflection is generated due to surface plasmon resonance can be obtained, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of surface plasmon resonance, i.e., a state of attenuation in total internal reflection.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface changing the angle of incidence or a relatively thick incident light beam may be caused to impinge upon the interface in the form of convergent light or divergent light so that the incident light beam includes components impinging upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the angle of incidence changes may be detected by a photodetector which is moved in synchronization with the change of the angle of incidence or by an area sensor extending in the direction in which reflected light beam is moved as the angle of incidence changes. In the latter case, an area sensor which extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.
In such a surface plasmon resonance sensor, when a light beam impinges upon the metal film at a particular angle of incidence θsp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution in the sample in contact with the metal film are generated and surface plasmon is excited in the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total internal reflection at the interface of the dielectric block and the metal film sharply drops. The sharp intensity drop is generally detected as a dark line by the photodetector.
The aforesaid resonance occurs only when the incident light beam is p-polarized. Accordingly, it is necessary to set the surface plasmon sensor so that the light beam impinges upon the interface in the form of p-polarized light or p-polarized components are only detected.
When the wave number of the surface plasmon can be known from the angle of incidence θsp at which the phenomenon of attenuation in total internal reflection (ATR) takes place, the dielectric constant of the sample can be obtained. That is
            K      sp        ⁡          (      ω      )        =            ω      c        ⁢                                                      ɛ              m                        ⁡                          (              ω              )                                ⁢                      ɛ            s                                                              ɛ              m                        ⁡                          (              ω              )                                +                      ɛ            s                              wherein Ksp represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in a vacuum, and ⊂m and ∈s respectively represent the dielectric constants of the metal and the sample.
When the dielectric constant ∈s of the sample is known, the concentration of a specific material in the sample can be determined on the basis of a predetermined calibration curve or the like. Accordingly, a property related to the dielectric constant (refractive index) of the sample can be detected by detecting the angle of incidence θsp at which the intensity of light reflected in total internal reflection from the interface of the prism and the metal film sharply drops (this angel θsp will be referred to as “the attenuation angle θsp”, hereinbelow).
As a similar apparatus utilizing the phenomenon of attenuation in total internal reflection (ATR), there has been known a leaky mode sensor described in, for instance, “Surface Refracto-sensor using Evanescent Waves: Principles and Instrumentations” by Takayuki Okamoto (Spectrum Researches, Vol. 47, No. 1 (1998), pp. 21 to 23 & pp. 26 and 27). The leaky mode sensor basically comprises a dielectric block shaped, for instance, like a prism, a clad layer which is formed on one face of the dielectric block, an optical waveguide layer which is formed on the clad layer and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block at various angles of incidence so that total internal reflection conditions are satisfied at the interface of the dielectric block and the clad layer and various angles of incidence of the light beam to the interface of the dielectric block and the clad layer including an angle of incidence at which attenuation in total internal reflection is generated due to optical waveguide mode excitation can be obtained, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of waveguide mode excitation, i.e., a state of attenuation in total internal reflection (ATR).
In the leaky mode sensor with this arrangement, when the light beam is caused to impinge upon the clad layer through the dielectric block at an angle not smaller than an angle of total internal reflection, only light having a particular wave number and impinging upon the optical waveguide layer at a particular angle of incidence comes to propagate through the optical waveguide layer in a waveguide mode after passing through the clad layer. When the waveguide mode is thus excited, almost all the incident light is taken in the optical waveguide layer and accordingly, the intensity of light reflected in total internal reflection at the interface of the dielectric block and the clad layer sharply drops. That is, attenuation in total internal reflection occurs. Since the wave number of light to be propagated through the optical waveguide layer in a waveguide mode depends upon the refractive index of the sample on the optical waveguide layer, the refractive index and/or the properties of the sample related to the refractive index can be detected on the basis of the attenuation angle θsp at which the attenuation in total internal reflection occurs.
There have been known various types of measuring apparatuses such as a surface plasmon resonance sensor or a leaky mode sensor utilizing the phenomenon of attenuation in total internal reflection (ATR). The type of such measuring apparatuses depends upon the manner in which the properties of the sample are analyzed on the basis of the state of light reflected by the interface which changes with generation of the evanescent waves. In one type, the attenuation angle θsp is measured, in another type, light beams of different wavelengths are caused to impinge upon the interface and the degree of attenuation in total internal reflection is detected by the wavelength, and in still another type, a main light beam is caused to impinge upon the interface and the light beam reflected by the interface is caused to interfere with a light beam split from the main light beam before it impinges upon the interface, thereby measuring the state of interference.
In order to perform measurement on a number of samples at high speed, and to increase efficiency of handling, there has been proposed such a measuring apparatus in which a sensor well unit comprising a dielectric block having a plurality of one-dimensionally or two-dimensionally arranged sample wells open in the upper surface thereof is used, and a plurality of light beams are caused to impinge upon the sample wells in parallel, thereby separately detecting the light beams reflected by the interfaces for the respective sample wells.
In the surface plasmon resonance sensor or the leaky mode sensor of the type described above, it is sometimes necessary to perform measurement a plurality of times on a sample at intervals and to detect the change of the state. In such a case, in order to perform such measurement on a plurality of samples at high efficiency, a sensor well unit is once demounted from the measuring apparatus after a first measurement on the sample placed in its sample well and then mounted again on the measuring apparatus after measurement on the samples placed in the sample wells of another one or more sensor well units. Conventionally, there has been a problem that the position of the base line (the interface) changes each time the same sensor well unit is mounted on the measuring apparatus, that is, the preceding base line is tilted to the next base line. When the tilt of the base line is in a longitudinal direction in which the angle of incidence of the light beam to impinge upon the base line is changed, the angle of reflection of the detecting light beam is also shifted, which deteriorates the accuracy of measurement.
Also tilt of the base line in the transverse direction intersecting the longitudinal direction can result in that the reflected light beam can travel in a direction in which it cannot be received by the photodetector. That is, tilt of the interface in both the longitudinal direction and the transverse direction can lead to deterioration of the accuracy of measurement.